Luis is 4 years younger than Stephanie. For the last two years, Stephanie and Luis have been going to the same school. Seven years ago, Stephanie was 3 times as old as Luis. How old is Stephanie now?
Explanation: We can use the given information to write down two equations that describe the ages of Stephanie and Luis. Let Stephanie's current age be $s$ and Luis's current age be $l$ The information in the first sentence can be expressed in the following equation: $s = l + 4$ Seven years ago, Stephanie was $s - 7$ years old, and Luis was $l - 7$ years old. The information in the second sentence can be expressed in the following equation: $s - 7 = 3(l - 7)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $s$ , it might be easiest to solve our first equation for $l$ and substitute it into our second equation. Solving our first equation for $l$ , we get: $l = s - 4$ . Substituting this into our second equation, we get the equation: $s - 7 = 3($ $(s - 4)$ $ -$ $ 7)$ which combines the information about $s$ from both of our original equations. Simplifying the right side of this equation, we get: $s - 7 = 3s - 33$ Solving for $s$ , we get: $2 s = 26$ $s = 13$.